Classic PCB Thermal Profiling of the Reflow Solder Process -- Are you throwing profits away? If you're not profiling your PCB assemblies, you might want to rethink your assembly process.
Classic thermal profiling of PCBs will ensure optimum, consistent quality of finished PCB assemblies; substantially reduce PCB scrap rates; improve PCB production rates and yields; and improve overall profitability.
The Reflow Process
During the reflow process, heat is applied within the oven to bring the assembly to the proper soldering temperatures without damaging the product. To verify the process of reflow soldering, a profiling device is used to determine process setup. A profile is a visual data log of the time and temperature of each sensor as it progresses through the heating process. By reviewing this profile, one can visually see exactly how much energy, and where the energy, is being applied to the product. The profile allows the operator to make appropriate changes to optimize the reflow process.
A typical reflow profile contains several different stages-initial ramp, soak, spike to reflow, reflow and cool down of the product. As a general rule, temperature slopes in the range of 2 to 4 degrees C are desirable to prevent damage to the board and/or components caused by heating or cooling too rapidly.
During heating of the product, many factors can affect the quality of the assembly. The initial ramp is a quick rise in temperature as the product enters the oven. The objective is to bring the paste to the desired soak temperatures required to begin the activation of the solder. The most desirable soak temperature is just under the melting point of the paste material-183 degrees C for eutectic solders, with a soak time of between 30 and 90 seconds. The soak zone has two purposes: 1) to bring the board, components and material to an even temperature, close to the melting point of the solder paste, to allow for an easier transition into the reflow zone and 2) to activate the flux on the assembly. At soak temperatures, activated flux begins the process of cleaning the pads and leads of oxidation, leaving clean surfaces to which the solder can adhere. Spike to reflow is another transition in which the assembly temperature rises above the melting point of the solder and a liquidous state of the paste occurs.
Once the solder paste is above the melting point, the assembly enters the reflow zone, commonly referred as the time above liquidous (TAL). The reflow zone is a critical state within the oven, as the temperature gradients on the assembly must be minimized and the TAL must be kept within the parameters specified by the paste manufacturer. The peak temperature of the product is also reached during this period-the point at which the assembly reaches the maximum temperature within the oven.
Caution must be used not to exceed maximum temperatures and heating rates of any temperature-sensitive components on the board. For example, a typical tantalum capacitor has a maximum temperature rating of 230 degrees C. Ideally, all points on the assembly should reach the same peak temperature, at the same time and rate, to ensure that all parts are subjected to an identical environment within the oven. After the reflow zone, the product is cooled to solidify the solder joints and prepare the assembly for future processes. Controlling the cooling slope is also critical, as cooling too quickly may damage the assembly and cooling too slowly will increase the TAL and may cause brittle solder joints.
Two common types of profiles are used in the reflow soldering process. The profiles are commonly referred to as the soak and tent profiles. During the soak profile (Figure 1), as described above, the assembly is subjected to the same temperature for a period of time. The tent profile (Figure 2) is a continuous ramp of temperature from the time the assembly enters the oven until the assembly reaches the desired peak temperature.
Desired profiles will differ based on the type of solder paste used in the building of the assembly. Depending upon the chemical makeup of the solder paste, the manufacturer will suggest the best profile to achieve maximum performance. Profile information may be found by contacting the paste manufacturer. The most common types of formulations include water soluble (OA), rosin mildly activated (RMA) and no-clean solder pastes.
The Mechanics of Profiling
Classic PCB Profiling System Components
A classic PCB profiling system consists of the following components:
-A data-collecting profiling instrument that travels through the oven gathering thermal information from the PCB.
- Thermocouples that are attached to critical components on the PCB and then connected to the traveling profiling instrument.
- A protective thermal barrier to hold and protect the profiler from the heat inside the oven.
- A software program that allows collected data to be viewed in a format that quickly verifies solder results and/or identifies out-of-control trends before they adversely affect the final PCB product.
Type K thermocouples are the most commonly used in the electronics industry. A variety of techniques exist to attach thermocouples to the components of a PCB. The method used depends upon the type of PCB being processed, as well as user preference.
High-temperature solder provides a very strong physical connection to the PCB. This method is commonly used in an operation that can afford to sacrifice a dedicated reference board for profiling and process verification. Caution should be used to ensure that a minimum amount of solder is applied to avoid affecting the profile.
Adhesives can be used to secure the thermocouple to the assembly. The use of adhesives usually results in a positive physical connection of the thermocouple junction to the assembly. Drawbacks include the possibility of the adhesive failing during the heating process, removal at the conclusion of the profile and residue left on the assembly. Again, caution should be taken to use the minimum amount of adhesive, as adding thermal mass can affect the results of the profile.
Kapton or aluminum tape is easy to use, but provides the least secure method of attachment. Profiles using tape will often show a rather jagged profile due to the thermocouple junction lifting from the contact surface during heating. Ease of use, and lack of residual or destructive effects to the assembly, make Kapton or aluminum tape a popular method.
Pressure type thermocouples clamp to the edge of a circuit board and use spring tension to hold a thermocouple junction firmly in contact with the assembly being profiled. Pressure probes are quick and easy to use and are non-destructive to the PCB.
Because the outside edges and corners of an assembly will heat up faster than the center, and components of greater thermal mass will heat more slowly than components of lesser thermal mass, a minimum of four thermocouple placements is recommended. One thermocouple should be placed toward an edge or corner of the assembly being profiled, one on a small component, another toward the center of the board and a fourth on a component of higher mass. Most profilers have five or six channels. Additional thermocouples can be placed on other parts of the board of interest or on components at risk to thermal shock or high temperature damage.
Reading and Evaluating Profiling Data
Solder paste manufacturers typically have profile recommendations specific to their paste formulations. Manufacturer recommendations should be used to determine the best profile for a particular process and compared against actual assembly results. Steps may then be taken to alter machine settings to deliver the best results for the specific assembly (Figure 3).
New tools are now available to PCB assemblers that make designing a target profile for a specific combination of solder paste and reflow oven easy. Once designed, this target profile can simply be called up by the machine operator and set to automatically run on the reflow oven based on that specific PCB assembly.
When to Profile
Profiling is especially useful when starting with a new assembly. Oven settings must be determined to optimize the process for high-quality results. As a diagnostic tool, a profiler is invaluable in helping determine the causes of poor yields and/or high rework.
Profiling can uncover inappropriate oven settings or assure that settings are appropriate to the assembly. Many facilities run a profile on standard reference boards or with a machine quality management profiling instrument everyday. Some facilities run profiles at the beginning of each shift to verify oven operation and avoid potential problems before they happen. These profiles can be stored as a hard copy, or via electronic format, and used as part of an ISO program or to perform statistical process control (SPC) operations on machine performance over time.
Care should be taken with the assembly used for profiling. The assembly may degrade over time from mishandling or repeated exposure to reflow temperatures. The board of a profiling assembly can delaminate over time and thermocouple attachments can loosen, which should be expected and accounted for by examining the profiling device prior to each run for any damage that may skew the results. The key is to ensure that the measuring device is capable of delivering accurate results.
Classic PCB Profiling Versus Machine Quality Management Profiling
While the most popular type of thermal profiling involves using a traveling profiler and thermocouples to monitor PCB component temperatures, profiling is also used to ensure that the reflow oven is continually operating at optimal settings. A variety of self-contained machine profiling instruments are available to provide a daily check of key reflow oven parameters, including air temperature, heat flow and conveyor speed. These instruments also provide the opportunity to quickly identify any out-of-control trends before they adversely affect the quality of the finished PCB assembly.
Thermal profiling is a key element in the assembly of PCBs, both to determine process machine settings and to verify process consistency. Without measurable results, control over the reflow process is limited. Consult paste vendors and review component specifications to determine the best profile parameters for a particular process. By implementing a regular regimen of both classic PCB profiling and machine quality management profiling, PCB scrap rates will be lowered while quality and throughput is improved. As a result, total operating costs will be reduced.
Andy Becker is vice president of ECD Instruments Group, Milwaukie, OR;
e-mail: firstname.lastname@example.org. Marc C. Apell is reflow products manager for Speedline Technologies ELECTROVERT, Camdenton, MO; e-mail: email@example.com.
Profiling the Wave Solder Process
While this article focuses on the reflow solder process, classic PCB profiling can also be performed on assemblies as they pass through a wave solder machine. Techniques and profiling benefits are similar to those utilized and obtained for the reflow process. In addition, a variety of self-contained profiling instruments, designed to collect data from the wave solder machine to quickly identify any out-of-control trends and monitor consistent day-to-day and shift-to-shift operations, may be selected.
This instrument allows operators of wave solder machines to perform routine operation checks and troubleshooting diagnostics by measuring conveyor speed, solder wave and pre-heat parameters.
Copyright [copyright] 2001 Miller Freeman LLC
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|Title Annotation:||printed circuit boards; Profiling Primer|
|Author:||Apell, Marc C.|
|Date:||May 1, 2001|
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